1. Field of the Invention
The present invention relates to a data compression system for compressing given data by a coder.
In facsimiles and optical disk file systems etc., if data obtained by reading document data is sent to the other party or stored in an output file as is, the amount of data transmitted to the other party or the amount of stored data of the output file becomes tremendous. Therefore, the method used for a long time now has been to process the above given data to enable considerably faithful reproduction of the original given data even with a small amount of data transmission or a small amount of data storage. This processing is called data compression.
2. Description of the Related Art
In the past, use has been made of the Modified Huffman coding system (MH coding system) as an internationally standard one-dimensional compression system of binary picture image data and the Modified Modified Relative address designate coding system (MMR coding system) as an internationally standard two-dimensional compression system of binary picture image data. Further, there are the predictive coding system and the universal coding system, as represented by the Ziv-Lempel code, for compression of character code data.
Details of these above various types of data compression systems are given later, but there are disadvantages to the data compression systems, as follows:
i) In the MH coding system, the degree of data compression is low, so sufficient data compression cannot be expected with this system alone.
ii) In the MMR coding system, there are two disadvantages:
The first disadvantage is that since the MMR system codes changing picture elements one by one, when the resolution rises, there is the problem that the amount of the codes increases substantially in proportion to the resolution. For example, when the resolution rises twice as high, the number of picture elements in the main scanning direction doubles and the number of sub scanning lines double. The number of changing picture elements on the sub scanning line remains substantially the same as before the resolution rises, so the number of changing picture elements substantially doubles and the amount of codes becomes about twice as much. It may be considered that the inherent amount of data of the picture does not increase in proportion to the resolution. Along with the rise in the resolution, the MMR system features a reduction in the compression efficiency with respect to the inherent amount of the picture image data.
The second disadvantage is that shaded pictures become screen dot pictures in digital pictures and the halftones are expressed by the area density of black picture elements. Screen dot pictures involve vast numbers of changing picture elements as the screen dots are dispersed over the screen as a whole, so in the MMR system, effective compression was not possible.
iii) In the predictive coding system, there were the following two disadvantages:
The first disadvantage was that when the resolution increased, predictive functions were required corresponding to each resolution and if the predictive functions were used as they were, the efficiency of prediction would fall and a sufficient compression ratio could not be obtained.
The second disadvantage was that, in screen dot pictures, compression was possible if use was made of predictive functions tailored to the screen dot pictures for compression, but when compressing screen dot pictures of various *cycles and shapes, compression was not possible with predictive functions tailored to specific screen dots.
iv) In the universal coding system, a high data compression ratio could be obtained, but the amount of calculation required for the processing for the compression became larger.